High stiffness high impact propylene impact copolymers field of the invention

ABSTRACT

The present invention relates to polypropylene impact copolymer compositions which exhibit improved stiffness without degrading the impact resistance performance. The polypropylene impact copolymer comprises a matrix and a dispersed phase. The matrix comprises a polypropylene homopolymer or a propylene/alpha-olefin random copolymer which comprises more than 50 wt. % of units derived from propylene monomer. The matrix should have a relatively high crystallinity, preferably 50% or greater. The polypropylene homopolymer or a propylene/alpha-olefin random copolymer preferably has a MWD between 4 and 8, such as typically obtained using Ziegler-Natta catalysts. The dispersed phase in the impact copolymer comprises an ethylene-propylene copolymer which comprises from 45 to 70 wt. % of units derived from an ethylene monomer. Preferably the dispersed phase comprises from 20 to 50 percent by weight of the polypropylene impact copolymer.

FIELD OF THE INVENTION

The present invention relates to polypropylene impact copolymers havingimproved stiffness and good impact properties.

BACKGROUND AND SUMMARY OF THE INVENTION

Propylene impact copolymers (ICP's) are commonly used in a variety ofapplications where strength and impact resistance are desired such asmolded and extruded automobile parts, household appliances, luggage andfurniture.

Propylene homopolymers or propylene based random copolymers having highcrystallinity are often unsuitable for such applications by themselvesbecause they are too brittle and have low impact resistance, whereaspropylene impact copolymers are specifically engineered for applicationssuch as these.

Propylene impact copolymers are typically an intimate mixture of acontinuous phase of crystalline propylene homopolymer or randomcopolymer and dispersed rubbery phase of ethylene-propylene copolymer.In general the continuous phase is known to provide properties such asstiffness and the dispersed phase provides impact resistance properties.

In general, it has been observed that the properties of stiffness, andimpact resistance trend in opposite directions such that as stiffnessincreases, impact resistance decreases and vice versa. It would bedesirable to develop compositions which exhibit improved stiffnesswithout degrading the impact resistance performance.

The present invention relates to such a composition. Specifically, oneaspect of the present invention is a polypropylene impact copolymercomprising: a matrix and a dispersed phase. The matrix comprises apolypropylene homopolymer or a propylene/alpha-olefin random copolymerwhich comprises more than 50 wt. % of units derived from propylenemonomer. The matrix should have a relatively high crystallinity,preferably 50% or greater. The polypropylene homopolymer or apropylene/alpha-olefin random copolymer preferably has a MWD between 4and 8, such as typically obtained using Ziegler-Natta catalysts. Thedispersed phase in the impact copolymer comprises an ethylene-propylenecopolymer which comprises from 45 to 70 wt. % of units derived from anethylene monomer. Preferably the dispersed phase comprises from 20 to 50percent by weight of the polypropylene impact copolymer

DETAILED DESCRIPTION OF THE INVENTION Analytical Methods:

Unless otherwise indicated, the following analytical methods are used inthe present invention:

Flexural modulus is determined in accordance with ASTM D790-00 Method 1,using an ASTM D 638 specimen tested at 1.3 mm/min.

Molecular weights (Mn, Mw and Mz) and molecular weight distributionsMw/Mn (also referred to as “MWD”) and Mz/Mw are measured by GPCaccording to the Gel Permeation Chromatography (GPC) Analytical Methodfor Polypropylene. The polymers are analyzed on a PL-220 series hightemperature gel permeation chromatography (GPC) unit equipped with arefractometer detector and four PLgel Mixed A (20 μm) columns (PolymerLaboratory Inc.). The oven temperature is set at 150° C. and thetemperatures of autosampler's hot and the warm zones are at 135° C. and130° C. respectively. The solvent is nitrogen purged1,2,4-trichlorobenzene (TCB) containing ˜200 ppm2,6-di-t-butyl-4-methylphenol (BHT). The flow rate is 1.0 mL/min and theinjection volume was 200 μl. A 2 mg/mL sample concentration is preparedby dissolving the sample in N2 purged and preheated TCB (containing 200ppm BHT) for 2.5 hrs at 160° C. with gentle agitation.

The GPC column set is calibrated by running twenty narrow molecularweight distribution polystyrene standards. The molecular weight (MW) ofthe standards ranges from 580 to 8,400,000 g/mol, and the standards werecontained in 6 “cocktail” mixtures. Each standard mixture has at least adecade of separation between individual molecular weights. Thepolystyrene standards are prepared at 0.005 g in 20 mL of solvent formolecular weights equal to or greater than 1,000,000 g/mol and 0.001 gin 20 mL of solvent for molecular weights less than 1,000,000 g/mol. Thepolystyrene standards are dissolved at 150° C. for 30 min understirring. The narrow standards mixtures are run first and in order ofdecreasing highest molecular weight component to minimize degradationeffect. A logarithmic molecular weight calibration is generated using aforth-order polynomial fit as a function of elution volume. Theequivalent polypropylene molecular weights are calculated by usingfollowing equation with reported Mark-Houwink coefficients forpolypropylene (Th. G. Scholte, N. L. J. Meijerink, H. M. Schoffeleers,and A. M. G. Brands, J. Appl. Polym. Sci., 29, 3763-3782 (1984)) andpolystyrene (E. P. Otocka, R. J. Roe, N. Y. Hellman, P. M. Muglia,Macromolecules, 4, 507 (1971)):

$M_{PP} = ( \frac{K_{PS}M_{PS}^{a_{PS} + 1}}{K_{PP}} )^{\frac{1}{a_{PP} + 1}}$

where M_(pp) is PP equivalent MW, M_(PS) is PS equivalent MW, log K anda values of Mark-Houwink coefficients for PP and PS are listed below inTable 1.

TABLE 1 Polymer A log K Polypropylene 0.725 −3.721 Polystyrene 0.702−3.900

Izod impact strength is measured in accordance with ASTM D 256.

Melt flow rate (MFR) is measured in accordance with ASTM D 1238-01 testmethod at 230° with a 2.16 kg weight for propylene-based polymers.

Xylene Solubles (XS) is measured according to the following procedure:0.4 g of polymer is dissolved in 20 ml of xylenes with stirring at 130°C. for 30 minutes. The solution is then cooled to 25° C. and after 30minutes the insoluble polymer fraction is filtered off. The resultingfiltrate is analyzed by Flow Injection Polymer Analysis using a ViscotekViscoGEL H-100-3078 column with THF mobile phase flowing at 1.0 ml/min.The column is coupled to a Viscotek Model 302 Triple Detector Array,with light scattering, viscometer and refractometer detectors operatingat 45° C. Instrument calibration was maintained with Viscotek PolyCAL™polystyrene standards.

Melting point is determined by DSC, ASTM D3418.

Heat resistance (HDT) is determined according to ASTM D648.

Et (total ethylene wt. % in the propylene impact copolymer) is measuredby a well known method reported by S. Di Martino and M. Kelchtermans“Determination of the Composition of Ethylene-Propylene Rubbers Using13C-NMR Spectroscopy” J. of Applied Polymer Science, v 56, 1781-1787(1995).

The amorphous rubber content in the impact copolymer generally can beassessed by dissolving the impact copolymer in xylene. The amount ofxylene solubles measured by the Viscotek method (described above) plus 2wt. % corresponds to the amount of dispersed rubber phase (Fc) in theimpact copolymer.

Ec (ethylene content wt % in the dispersed phase) is calculated asEc=Et*100/Fc.

The propylene impact copolymers (sometimes referred to as “ICPs”) ofthis invention comprise at least two major components, the matrix andthe dispersed phase. The matrix is preferably an isotactic propylenehomopolymer, though small amounts of a comonomer may be used to obtainparticular properties. Typically such copolymers of the matrix contain10% by weight or less, preferably less than 6% by weight or less,comonomer such as ethylene, butene, 1-hexene or 1-octene. Mostpreferably less than 4% by weight ethylene is used. The inclusion ofcomonomer typically results in a product with lower stiffness but withhigher impact strength compared to impact copolymers where the matrix ishomopolymer polypropylene.

The characteristics of the matrix of the impact copolymers can generallybe determined from an analysis of the xylene insoluble portion of theimpact copolymer composition, while the characteristics of the dispersedphase are attributable to the xylene soluble portion.

The polymer material used in the matrix of the impact copolymers of thepresent invention preferably has a relatively broad molecular weightdistribution Mw/Mn (“MWD”), i.e., 4.0 to about 8, preferably greaterthan 4 to about 7, more preferably from 4.5 to 6. These molecular weightdistributions are obtained in the absence of visbreaking using peroxideor other post reactor treatment designed to reduce molecular weight. Ingeneral polymers having a higher MWD results in impact copolymers havinggreater stiffness but less impact resistance.

The matrix polymer preferably has a weight average molecular weight (Mwas determined by GPC) of at least 200,000, preferably at least 300,000and a melting point (Mp) of at least 145° C., preferably at least 155°C., more preferably at least 152° C., and most preferably at least 160°C.

Another important parameter of the matrix polymer is the amount ofxylene solubles (XS) they contain. The matrix polymer of this inventionare characterized as having low XS, preferably less than 3% by weight,more preferably less than 2% by weight, even more preferably less than1.5% by weight.

The dispersed phase for use in the impact copolymers of the presentinvention comprises propylene/ethylene copolymer where thepropylene/ethylene copolymer is comprised of from 45 to 70 wt. % ofunits derived from an ethylene monomer. More preferably thepropylene/ethylene copolymer is comprised of from 50 to 65 wt. % ofunits derived from an ethylene monomer. In some application it may bepreferably that the propylene/ethylene copolymer comprises more than 50%of units derived from an ethylene monomer.

The propylene/ethylene copolymer for use as the dispersed phase in thepresent invention preferably has a molecular weight distribution Mw/Mn(“MWD”), of at least 2.5, preferably 3.5, and most preferably 4.5 orhigher. These molecular weight distributions should be obtained in theabsence of visbreaking or peroxide or other post reactor treatmentdesigned to reduce molecular weight. The propylene/ethylene copolymerpreferably has a weight average molecular weight (Mw as determined byGPC) of at least 100,000, preferably at least 150,000, and mostpreferably at least 200,000.

While these impact polypropylene products can be produced by meltcompounding the individual polymer components, it is preferred that theyare made in reactor. This is conveniently accomplished by polymerizingpropylene in a first reactor and transferring the high crystallinepolypropylene from the first reactor into a secondary reactor wherepropylene and ethylene are copolymerized in the presence of the highcrystalline material. Such “reactor-grade” products, theoretically canbe interpolymerized in one reactor, but are more preferably formed usingtwo reactors in series. The final impact copolymers as obtained from thereactor or reactors, however, can be blended with various othercomponents including other polymers.

The preferred melt flow rate (“MFR”) of the impact copolymers of thepresent invention depends on the desired end use but is typically in therange of from about 0.2 dg/min to about 200 dg/min, more preferably fromabout 5 dg/min to about 100 dg/min. Significantly, high MFRs, i.e.,higher than 50 dg/min, are obtainable. MFR is determined by aconventional procedure such as ASTM-1238 Cond. L (230° C./2.16 kg).Another known route to a high MFR product involves chemical treatment,i.e., visbreaking (peroxide treatment) of a molten PP heterophasiccopolymer. The impact copolymers of the present invention generallycomprise from about 50% to about 80% by weight of the matrix and fromabout 20% to about 50% by weight of the dispersed phase, preferably fromabout 60% to about 70% by weight of the matrix and from about 30% toabout 40% of the dispersed phase.

The overall comonomer (preferably ethylene) content of the total impactcopolymer is preferably in the range of from about 10% to about 35% byweight, more preferably from about 12% to about 28% by weight, even morepreferably from about 15% to about 25% by weight comonomer.

A variety of additives may be incorporated into the impact copolymer forvarious purposes as is generally known in the art. Such additivesinclude, for example, stabilizers, antioxidants, fillers, colorants,nucleating agents and mold release agents.

The impact copolymers of this invention may conveniently be prepared byconventional polymerization processes such as a two-step processalthough it is conceivable that they may be produced in a singlereactor. Each step may be independently carried out in either the gas orliquid slurry phase. For example the first step may be conducted in agas phase or in liquid slurry phase. Preferably the impact copolymers ofthis invention are produced in multiple reactors, preferably two orthree, operated in series. The dispersed phase is preferably polymerizedin a second, gas phase reactor.

In an alternative embodiment, the polymer material used for the matrixis made in at least two reactors in order to obtain fractions withvarying melt flow rate. This has been found to improve theprocessability of the impact copolymers.

As is generally known in the art, hydrogen may be added to any of thereactors to control molecular weight, intrinsic viscosity and melt flowrate (MFR). The composition of the dispersed rubber phase is controlled(typically in the second reactor) by the ethylene/propylene ratio andthe amount of hydrogen.

By way of example and not by limitation, examples of the presentdisclosure will now be provided.

EXAMPLES

A first series of propylene impact copolymers was made in a dual reactorset up where the matrix polymer was made in a first gas phase reactorand then the contents of the first reactor are passed to a second gasphase reactor. The ethylene content in the rubber phase (Ec) and theamount of the dispersed phase (Fc) for each ICP is reported in Table 1below. Typical reaction conditions are used to make these impactco-polymers. The melt flow rate of the granules after the second reactorfor each of these materials is about 1.4 g/10 min. The powder granulesare cracked so that the cracked material has a target melt flow rate ofaround 8 g/10 minutes, using Trigonox301 as a peroxide-containingvisbreaking agent in a 30 mm co-rotating twin screw extruder. A typicalanti-oxidant package and nucleator agent Sodium Benzoate 800 ppm areadded into all the examples during visbreaking.

The pellet samples are injection molded according to ASTM D4101 with a4-cavity family mold. The resulting molded dog-bone is used for flex andIzod testing. The resulting molded straight bar is used for heatresistance (HDT) testing according to ASTM D648. The results of thesetests are presented in Table 1.

As seen from the Table, Examples 1 and 2 have better Flexual modulus,Izod impact, and heat resistance (HDT) than comparative examples 1 and 2in most situations.

TABLE 1 Examples 1 2 Comp. 1 Comp. 2 MF (g/10 min), 1.5 1.4 1.3 1.4 Rx 2MF (g/10 min), 10.3 6.5 9.9 7.7 final Ec, wt. % 52 56 40 46 Fc, Wt % 3637 35 35 Flex, Kpsi, 1% 130 125 113 119 Secant modulus N. Izod, ft-lb/in14.7 14.8 13.5 13.6 @ 23° C. N. Izod, ft-lb/in 13.7 14.1 13.5 13.7 @ 0°C. N. Izod, ft-lb/in 12.7 13.9 12.8 13.0 @ −20° C. HDT, ° C. 85 85 78 80

A second series of propylene impact copolymers are made as describedabove except that the powder granules are cracked to around 12 MF asseen from Table 2. Preparation of materials for testing and the teststhemselves are conducted as set forth above for Table 1. The results ofthese tests are presented in Table 2.

As seen from that table, Examples 3 and 4 have better Flexual modulus,Izod impact, and heat resistance (HDT) than the comparative examples 3and 4 in most situations.

TABLE 2 Examples 3 4 Comp. 3 Comp. 4 MF (g/10 min), 1.5 1.4 1.3 1.4 Rx 2MF (g/10 min), 12.1 11.2 13.1 12 final Ec, wt. % 52 56 40 46 Fc, Wt % 3637 35 35 Flex, Kpsi, 1% 127 121 112 117 Secant modulus N. Izod, ft-lb/in13.5 13.6 13.2 13.4 @ 23° C. N. Izod, ft-lb/in 12.8 13.8 13.2 13.3 @ 0°C. N. Izod, ft-lb/in 11.4 12.9 12 12.2 @ −20° C. HDT, ° C. 80 81 77 80

1. A polypropylene impact copolymer comprising: (a) a matrix comprisinga polypropylene homopolymer or a propylene/alpha-olefin random copolymerwhich comprises more than 50 wt. % of units derived from propylenemonomer, the polypropylene homopolymer or a propylene/alpha-olefinrandom copolymer having an MWD between 4 and 8; wherein said matrix hasa crystallinity of at least 50%; (b) a dispersed phase comprising anethylene-propylene copolymer which comprises from 45 to 70 wt. % ofunits derived from an ethylene monomer; wherein the dispersed phasecomprises from 20 to 50 percent by weight of the polypropylene impactcopolymer.
 2. The propylene impact copolymer of claim 1 wherein thedispersed phase comprises from 30 to 40 percent by weight of thepolypropylene impact copolymer.
 3. The propylene impact copolymer ofclaim 1 wherein the matrix comprises a polypropylene homopolymer.
 4. Thepropylene impact copolymer of claim 1 wherein the matrix has an MWDbetween 4 and
 6. 5. The propylene impact copolymer of claim 1 whereinthe matrix is a propylene/alpha-olefin random copolymer which comprisesmore than 90 wt. % of units derived from propylene monomer.
 6. Thepropylene impact copolymer of claim 1 wherein the matrix is apropylene/alpha-olefin random copolymer in which the alpha olefin isethylene.
 7. The propylene impact copolymer of claim 1 wherein thematrix has a crystallinity greater than 60%.
 8. The propylene impactcopolymer of claim 1 wherein the dispersed polymer comprises at least 50wt. % of ethylene.
 9. The propylene impact copolymer of claim 1 having amelt flow rate of from 1 to 50 g/10 min (230° C./2.16 kg).
 10. Thepropylene impact copolymer of claim 1 which has been vis-broken and thevis-breaking ratio is greater than or equal to
 2. 11. The propyleneimpact copolymer of claim 10 in which the vis-breaking ratio is greaterthan or equal to 4.